Currently, breakthrough has been made in the engineering application of large capacity long distance DC transmission technology that plays an important role in the optimal allocation of energy resources. As the fully controlled switching device emerges and matures, the flexible DC technology with VSC (Voltage Source Converter) as its core component has become an important development trend for DC transmission and distribution, which can not only provide more reliable power supply, redundant system, adaptable power supply mode, flexible and secure flow control, but also effectively improve the impact of new energy on the safe and stable operation of poster grid. In general, the flexible DC transmission system adopts two-end or multi-end VSC DC parallel topology whose AC side is connected w parallel to the electrical system via a transformer.
As the scale and load of modern power grid continues to grow, it is more difficult to control the power floss and voltage of AC power grid; meanwhile, owing to the scarce land resources and enhanced environmental requirements, the newly constructed substation and transmission line have become more and more constrained. Therefore, new technologies are required to give lull play to the potentials of the existing power grid, for which the Flexible Alternative Current Transmission System (FACTS) has offered an important solution. As the highest level of FACTS technology at present, the Unified Power Flow Controller (UPFC) can perform flexible control on multiple electrical parameters through VSC control to regulate the active and reactive power of transmission lines and control the reactive voltage of the system. In general, the two VSC DC sides of UPFC are connected to each other back to back, among which one end is connected to the power gird in parallel, and the other end is connected to the power grid in series via a series transformer.